1. Field of the Invention
The invention relates to a data transmission method and a tachograph system for the transmission of digital messages between a velocity transmitter and a recording unit. The invention also relates to a tachograph system having a velocity transmitter and a recording unit.
2. Related Art
Tachographs or tachograph systems are usually monitoring appliances provided in a motor vehicle, particularly in a goods or passenger transportation vehicle, to allow monitoring of the activities of a driver of the vehicle. In this case, a tachograph has the function of recording, processing and storing driver data for activities of a driver of the vehicle in order to allow these activities to be monitored by an inspector.
Earlier tachographs with printable disks have been replaced by electronic tachographs with digital data storage, referred to as digital tachographs. In Europe, digital tachographs of this kind are also stipulated in newly licensed vehicles for goods transport, for example.
During operation of the vehicle, the tachograph system records, stores and processes vehicle operating parameters, particularly velocities and driving times, but also other information. In this case, velocity data is transmitted from a velocity transmitter to a recording unit, where they are evaluated and stored. On the one hand, this may involve the transmission of signal pulses, produced by a magnet sensor on a drive shaft, and correspond to one particular revolution of the drive shaft.
These signal pulses can accordingly be used to ascertain a velocity. In addition, it is also possible for a counter reading for the aforementioned signal pulses to be transmitted at particular times in order to keep this counter reading in sync in the velocity transmitter and in the recording unit.
To prevent manipulations in the transmission which feign an incorrect velocity, it is particularly possible for the counter reading to be transmitted using a cryptography method. Today, this involves the use of symmetric cryptography methods, with a symmetric session key being generated from module-individual information based on a general secret key, which is also called a master key. The general secret key is the same for all appliances and is known to the appliances only when a connection is set up for initializing purposes, what is known as pairing. If the secret master key is compromised, the integrity of all tachograph systems which use this key is endangered.